1. Field of the Invention
This invention pertains in general to the field of mining and, in particular, to a novel device and method for mining slurryable, shallow mineral deposits with or without earthy overburden in a longwall fashion.
2. Description of the Prior Art
Surface mining is and has historically been employed to recover stratified minerals under overburden to economic depths. Underground mining is traditionally employed when overburden depths exceed those economically removable by surface mining or when major surface disturbance is unacceptable.
Prior inventions have been patented for longwall mining of reserves using trenched entry where overburden is sufficiently competent to bridge over longwall shearing and conveying equipment and where floor strata are competent to withstand mining stresses. (See Simpson, U.S. Pat. No. 4,017,122.) Simpson does not accommodate soft, plastic, fluid, loose, unstable, clayey, sandy, dirt, soil, or similar (earthy) ground conditions often encountered in mining shallow ore deposits. Earthy conditions can allow the mine roof to fall ahead of shield supports or allow the floor to heave up behind the face conveyor ahead of the shield pontoons. This creates safety hazards, dilution of ores, and expensive control installation.
For surface mining and windrow reclaim, it has long been known to sluice the mining face or the mined earth to form a slurry for transport. A sluicing pit is created adjacent a mining pit mined with a dragline or other surface excavator. The excavator drops excavated earth into the sluicing pit. A water canon is provided at the sluicing pit to create a slurry, which is pumped away. As the excavator excavates, however, it moves farther away from the sluicing pit, increasing the time required to move the excavated earth to the pit, or requiring periodic reconstruction of the pit.
Some ores, such as phosphate bearing clay, are accompanied by a high degree of moisture. Traditional sluicing methods adds a significant amount of additional moisture, such that the solids content of the resulting slurry may be only 20 to 30%. Moving so much water is expensive. Moreover, in surface mining of such ores, the water tends to fill the mining pit making it difficult to retrieve the ore with excavating apparatus, such as the dragline.
Some ores, such as phosphate bearing clay, are contained within a horizontal plane that does not follow the inclination of the surrounding geologic strata. Prior art methods have not provided a convenient means for maintaining a horizontal mining plane.
The idea of adapting longwall mining equipment and methods to recover ore from slurryable deposits with earthy overburden is novel. The term xe2x80x9csoftwallxe2x80x9d is a new term applicable to this type of mining.
In particular, the subject invention is directed at phosphate matrix mining. A plurality of elongated, substantially parallel, main trenches extend the full length of area to be mined. The trenches are nominally 1,000 feet apart. Heading trenches substantially perpendicular to the main panel trenches are excavated for placement and removal of the mining equipment. The trenches are formed by excavating the overburden materials to the top surface of the mineral bed. The mineral bed in the trench is separately excavated and beneficially recovered. Trench side wall slopes are as steep as is geologically reasonable and safe to minimize excavation.
Forming a header trench leaves an exposed longwall. The softwall mining equipment is installed in the header trench. The phosphate is then mined, for example, by slurrying the ore as the mining equipment moves in a direction generally parallel to the main panel trenches. The slurried ore flows into the main panel trenches where it is removed to the surface for processing.
The softwall mining equipment includes an outer shell to support the overburden stresses. Forward motion is created by extending a cutting head into the ore reserve and retracting said head in such a manner as to pull the outer shell forward.
Unsupported overburden behind the outer shell is encouraged to fill the cavity. Where backfilling is used, materials are injected through the outer shell. Operation of the softwall equipment and backfilling is performed automatically from controls in the trench or on the surface.
When softwall mining equipment has traveled a predetermined distance to the next header trench, the equipment is removed and placed in another header trench for mining additional ore. Trenches not scheduled for further use would be reclaimed.
Alternatively, the equipment can be repositioned at the exit header and again advanced in the opposite direction to mine the next lower level of the ore seam.
Another alternative would be to utilize several sets of softwall mining equipment in a seam thicker than one set of equipment can mine. The uppermost level would be mined first. Adjacent lower levels would be mined with predetermined horizontal separation distances between sets of equipment.
Yet another alternative, where ore can be slumped, is to position the softwall mining equipment at or near the bottom of the ore seam. With or without forward injection of fluids into the ore seam, the slurried ore would slump into the softwall mining equipment and move into the main panel trenches.
Instead of using parallel main panel trenches and a common header trench, a single main trench can be used with a header constructed in a xe2x80x9cTxe2x80x9d manner. One set of softwall mining equipment would be placed in each header branch of the xe2x80x9cTxe2x80x9d with slurried ore feed to the trunk main panel trench.
The equipment can also operate in a spiral fashion following main panel trenches constructed to curl in a continuous pattern through the ore reserve.
In another aspect of the invention, individual softwall mining devices may be locked together to provide for advancing portions of the devices into the mining face without requiring the rear support typically provided by overburden for, e.g., surface mining and windrow reclaim.
In yet another aspect of the invention, selected softwall mining devices employed for, e.g., surface mining or windrow reclaim may be provided with a water canon or other water nozzle for preliminarily wetting the mining face or excavated earth.
Besides the objects and advantages described above, the softwall mining device of the present invention is also believed:
a. to provide a more economical means of mining slurryable ores;
b. to provide a means of removing ores by longwall methods where earthy overburden is present and where it is not;
c. to provide a means of longwall mining without use of panel development and outbye roof support;
d. to provide an alternative means of mining sticky clay ore; and
e. to provide a means of mining material varying from solid to liquid phases without special concern for the phase.